1. Field of the Invention
The present invention relates to a silver halide photosensitive material and a photothermographic material. More specifically, the invention relates to a silver halide photosensitive material and a photothermographic material which exhibit excellent image quality with a good degree of sharpness and little residual color.
2. Description of the Related Art
In recent years, decrease in the amount of processing liquid waste in the field of films for medical imaging has been keenly desired from the viewpoints of environmental protection and economy of space. For this reason, techniques regarding photothermographic materials for medical diagnosis and graphic arts, which can be exposed efficiently by laser image setters or laser imagers and can form clear black-toned images of high resolution and sharpness, are required. Thermal development systems which do not require liquid processing chemicals are simpler and do not damage the environment can therefore be supplied to customers.
While similar requirements also exist in the field of general image forming materials, images for medical imaging require a particularly high image quality excellent in sharpness and granularity since fine representation is required, and are characterized in that images of blue-black tones are preferred from the viewpoint of easy diagnosis. At present, various kinds of hard copy systems utilizing dyes or pigments such as ink jet printers and electrophotographic systems have been marketed as general image forming systems, but they are not satisfactory as output systems for medical images.
Thermal image forming systems utilizing organic silver salts are known. Particularly, a photothermographic material generally comprises an image forming layer in which a catalytically active amount of photocatalyst (for example, a silver halide), a reducing agent, a reducible silver salt (for example, an organic silver salt), and if necessary, a toner for controlling the color tone of silver, are dispersed in a binder. A photothermographic material forms a black silver image by being heated to a high temperature (for example, 80° C. or higher) after imagewise exposure to cause an oxidation-reduction reaction between a silver halide or a reducible silver salt (functioning as an oxidizing agent) and a reducing agent. The oxidation-reduction reaction is accelerated by the catalytic action of a latent image on the silver halide generated by exposure. As a result, a black silver image is formed in the exposed region. This system has been described in many documents, and the Fuji Medical Dry Imager FM-DP L is an example of a practical medical image forming system using a photothermographic material that has been marketed.
A thermal developing process for photothermographic materials does not require the processing solutions used in conventional wet processing, and has an advantage in that processing can be carried out easily and rapidly. However, there are problems to be solved in thermal developing process, which do not occur in conventional wet processing. One of them involves decolorizing dyes. Photosensitive materials commonly incorporate dyes in order to provide light filter and prevent halation or irradiation therein. The added dyes function during imagewise exposure. If the dyes remain in a photosensitive material after their use during exposure and development, the formed images may be colored thereby. Therefore the residual dyes must be removed from the photosensitive materials during the developing process. In a wet developing process, the residual dyes can be removed easily from the photosensitive materials by the processing solution. On the other hand, in the case of the thermal developing process, it is a difficult task to remove the residual dyes from the photosensitive materials.
More specifically, in order to attain images with a good degree of sharpness, the incorporation of dyes is very important for photosensitive materials exposed by a laser beam to provide sufficient antihalation and anti-irradiation effects over the wavelength region for the exposure. As for the wavelength of a laser beam used for the exposure, a wide range of wavelength regions such as the near infrared region, the infrared region, or the visible region from red to blue can be applied.
For photosensitive materials exposed imagewise by a near infrared laser beam or an infrared laser beam, dyes which have an absorption maximum within the near infrared or infrared region which are non-visible, a narrow half band width and little light absorption within the visible region are effectively applied. Japanese Patent Application Laid-Open (JP-A) Nos. 9-146220, and 11-228698 disclose photosensitive materials comprising such dyes described above, which require substantially no color bleaching mechanism.
However, in the case of a photosensitive material exposed by a laser beam of the visible region from blue to red, the inclusion of some color bleaching reaction mechanism therein is required.
Several methods to decolorize residual dyes upon heating during thermal a developing process have been proposed. For example, U.S. Pat. No. 5,135,842 discloses a decoloring method by heating for polymethine dyes having a specific structure. Moreover, U.S. Pat. Nos. 5,314,795, 5,324,627 and 5,384,237 disclose methods where polymethine dyes are decolorized by heating using a carbanion generating agent.
The inclusion of the discoloring mechanism described above may often bring about problems such as incomplete decoloring of dyes or dye decolorization during the storage of photothermographic materials due to the insufficient stability of dyes. Moreover, in the case where the polymethine dyes are used, the decomposition products of dyes remaining after a decoloring process have some light absorption within the visible region, whereby residual color in the image (especially in the highlight portion) may cause problems. Furthermore, the problem of recoloring after a thermal developing process (especially in contact with acids) and by-products formed by a complicated reaction mechanism may often worsen the handling properties of the photothermographic materials after processing.
Therefore, dye utilization techniques to solve the problems described above have been eagerly desired for photothermographic materials exposed by a laser beam of the visible spectrum regions or by a near infrared laser beam.